U.S. patent number 6,267,868 [Application Number 09/442,305] was granted by the patent office on 2001-07-31 for method and tool for electrochemical machining.
This patent grant is currently assigned to General Electric Company. Invention is credited to William Thomas Carter, Jr., Bruce Alan Knudsen, Hsin-Pang Wang, Bin Wei.
United States Patent |
6,267,868 |
Wei , et al. |
July 31, 2001 |
Method and tool for electrochemical machining
Abstract
An electrode for use in an electrochemical machining process
comprising an outer metal skin of corrosion resistant material, an
inner core of conductive material, and an insulating coating
disposed on an external surface of the outer metal skin. The
external surface is partially coated with the insulating coating so
as to define a pattern of raised areas to be formed on an internal
surface of a predrilled hole in a workpiece.
Inventors: |
Wei; Bin (Mechanicville,
NY), Knudsen; Bruce Alan (Amsterdam, NY), Carter, Jr.;
William Thomas (Galway, NY), Wang; Hsin-Pang (Rexford,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
26846900 |
Appl.
No.: |
09/442,305 |
Filed: |
November 22, 1999 |
Current U.S.
Class: |
205/648;
204/224M; 204/290.03; 204/290.08; 204/290.12; 204/290.13; 204/293;
205/652; 205/665; 205/670; 205/672 |
Current CPC
Class: |
B23H
3/04 (20130101); B23H 3/06 (20130101); B23H
9/00 (20130101); B23H 9/16 (20130101) |
Current International
Class: |
B23H
3/04 (20060101); B23H 9/16 (20060101); B23H
9/00 (20060101); B23H 3/06 (20060101); B23H
3/00 (20060101); B23H 011/00 () |
Field of
Search: |
;204/224M,290.01,290.03,290.12,290.13
;205/651,652,653,654,648,649,670,672,665,686 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Patnode; Patrick K. Snyder;
Marvin
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a provisional of Ser. No. 60/149,619, filed on
Aug. 16, 1999.
This application is related to commonly assigned B. Wei et al., "A
Method and Tool for Electrochemical Machining," U.S. Provisional
Application No. 60/149,616 B. Wei et al., "A Method and Tool for
Electrochemical Machining," U.S. Provisional Application No.
60/149,618 B. Wei et al., "A Method and Tool for Electrochemical
Machining," U.S. Provisional Application No. 60/149,617 each filed
Aug. 16, 1999 and are herein incorporated by reference.
Additionally, this application is related to commonly assigned B.
Wei et al., "A Method and Tool for Electrochemical Machining," U.S.
Application Ser. No. 09/187,663 and R. Guida et al., "Process for
Fabricating a Tool used in Electrochemical Machining," U.S.
application Ser. No. 09/187,664 each of which are herein
incorporated by reference and filed Nov. 5, 1998.
Claims
What is claimed is:
1. An electrode for use in an electrochemical machining process
comprising:
an outer metal skin of corrosion resistant material;
an inner core of conductive material; and
an insulating coating disposed on an external surface of said outer
metal skin wherein said external surface is partially coated with
said insulating coating so as to define a pattern of raised areas
to be formed on an internal surface of a predrilled hole in a
workpiece.
2. An electrode in accordance with claim 1, wherein said outer
metal skin is made of a titanium alloy.
3. An electrode in accordance with claim 1, wherein said outer
metal skin is made of a material selected from the group consisting
of Ti A40, Ti 64, and Ti 6242.
4. An electrode in accordance with claim 1, wherein said inner core
is made of a material selected from the group consisting of copper,
aluminum and silver.
5. An electrode in accordance with claim 1, wherein said insulating
coating is made of a material selected from the group consisting of
polymer, enamel and ceramic.
6. An electrode according to claim 1, wherein the pattern is
comprised of at least one ring circumferentially disposed on the
external surface of the cylinder.
7. An electrode according to claim 1, additionally comprising at
least one locator on said cylinder to position the electrode within
the hole.
8. An electrode according to claim 7, wherein said at least one
locator is disposed at a forward end of the cylinder and comprises
a coating of said insulating material thicker than the insulating
material in the pattern.
9. An electrode according to claim 8, wherein a second locator is
disposed at a midsection of the cylinder.
10. An electrode according to claim 1 wherein the pattern comprises
a plurality of spaced apart rings circumferentially disposed on the
external surface of the cylinder.
11. An electrochemical machining process for forming a raised area
in a wall of a predrilled hole in a workpiece comprising:
positioning in the hole an electrode having an outer metal skin of
corrosion resistant material; an inner core of conductive material;
and an insulating coating disposed on an external surface of said
outer metal skin wherein said external surface is partially coated
with said insulating coating so as to define a pattern of raised
areas to be formed on an internal surface of a predrilled hole in a
workpiece; and
machining at least one bulb in the wall of the hole by passing an
electric current between the electrode positioned in the hole and
the workpiece while circulating an electrolyte solution through the
hole.
12. A process according to claim 11 further comprising
simultaneously forming a plurality of raised areas in the surface
of the hole with the electrode.
13. A process according to claim 12 wherein said electrode is
stationarily positioned in said hole while said plurality of raised
areas are simultaneously formed with the electrode.
14. A process according to claim 11 wherein the hole has a
noncircular cross section and further comprising positioning the
electrode in a center of said hole.
15. A process according to claim 11 farther comprising positioning
the electrode in a center of said hole with a locator associated
with said electrode.
16. A process according to claim 11 wherein the electrolyte
solution is passed through the electrode into the hole.
17. A process according to claim 11 wherein the electrolyte
solution is passed through the hole around the electrode.
18. A process according to claim 11 wherein the workpiece comprises
a turbine blade and the raised area comprises a turbulator
ridge.
19. A turbine blade manufactured according to claim 18.
20. An electrode according to claim 1, wherein said pattern is a
helical pattern.
21. An electrode according to claim 1, wherein said pattern is a
spherical pattern.
Description
BACKGROUND OF THE INVENTION
This invention relates to a tool and a method used for
electrochemical machining. More particularly, this invention
relates to a tool and method for forming features in predrilled
holes using electrochemical machining.
A specialized adaptation of electrochemical machining, known as
shaped-tube electrochemical machining (STEM), is used for drilling
small, deep holes in electrically conductive materials. STEM is a
noncontact clectrochemical drilling process that can produce holes
with aspect ratios as high as 300:1. It is the only known method
which is capable of manufacturing the small, deep holes used for
cooling blades of efficient gas turbines.
The efficiency of a gas turbine engine is directly proportional to
the temperature of turbine gases channeled from the combustor of
the engine and flowing over the turbine blades. For example, for
gas turbine engines having relatively large blades, turbine gas
temperatures approaching 2,700.degree. F. are typical. To withstand
such high temperatures, these large blades are manufactured from
advanced materials and typically include state-of-the-art type
cooling features.
A turbine blade is typically cooled using a coolant such as
compressor discharge air. The blade typically includes a cooling
hole through which the air passes. A further design advancement has
been the addition of internal ridges in the cooling hole to effect
turbulent flow through the hole and increase cooling efficiency.
Cooling features within the hole such as turbulence promoting ribs,
or turbulators, thus increase the efficiency of the turbine.
The cooling holes commonly have an aspect ratio, or depth to
diameter ratio, as large as 300:1, with a diameter as small as a
few millimeters. The turbulators extend from sidewalls of the hole
into the air passage about 0.2 mm., for example.
The method currently used for drilling the cooling holes in turbine
blades is a shaped-tube electrochemical machining (STEM) process.
In this process, an electrically conductive workpiece is situated
in a fixed position relative to a movable manifold. The manifold
supports a plurality of drilling tubes, each of which are utilized
to form an aperture in the workpiece. The drilling tubes function
as cathodes in the electrochemical machining process, while the
workpiece acts as the anode. As the workpiece is flooded with an
electrolyte solution from the drilling tubes, material is deplated
from the workpiece in the vicinity of the leading edge of the
drilling tubes to form holes.
Turbulated ridges are formed in the cooling holes by a modification
of the standard shaped-tube electrochemical machining (STEM)
process for drilling straight-walled holes. One common method is
termed cyclic dwelling. With this technique, the drilling tube is
first fed forward, and then the advance is slowed or stopped in a
cyclic manner. The dwelling of the tool that occurs when the feed
rate is decreased or stopped creates a local enlargement of the
hole diameter, or a bulb. The cyclic dwelling, for which cyclical
voltage changes may be required, causes ridges to be formed between
axially spaced bulbs. These ridges are the turbulators.
The cyclic dwelling method is very low in process efficiency
compared to shaped-tube electrochemical machining (STEM) drilling
of straight-walled holes because of the lengthy required time for
drilling each bulb individually by cyclic tool dwelling. The dwell
time required to form a single bulb can be greater than the time
for drilling an entire straight-walled hole.
U.S. Pat. No. 5,306,401 describes a method for drilling cooling
holes in turbine blades that uses a complex tool resetting cycle
for each turbulator in the hole. This method also has low process
efficiency, having even longer operating times for drilling the
turbulator ridges than the cyclic dwelling method because of the
time required to reset the electrode tool.
In addition, both the cyclic dwelling method and the method
disclosed in U.S. Pat. No. 5,306,401 require that additional
equipment be used with a standard STEM machine for control of
machine ram accuracy, electrolyte flow and power supply
consistency, since these are crucial to hole quality. Failure to
control the dimensions of the turbulated holes often leads to part
rejection, adding significant manufacturing costs for the machining
process.
Accordingly, there is a need in the art for a new and improved
method for manufacturing turbulators that has a relatively short
machining cycle time. There is an additional need for an improved
method of manufacturing more complex features such as spiral or
helical ridges and the like. There is an additional need for a
method utilizing relatively simple and easily implemented
manufacturing techniques. In particular, there is a need for a
method that does not require complex lateral or vertical
displacement of the electrode.
SUMMARY OF THE INVENTION
An electrode for use in an electrochemical machining process
comprising an outer metal skin of corrosion resistant material, an
inner core of conductive material, and an insulating coating
disposed on an external surface of the outer metal skin. The
external surface is partially coated with the insulating coating so
as to define a pattern of raised areas to be formed on an internal
surface of a predrilled hole in a workpiece.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a conventional shaped-tube
electrochemical machining (STEM) electrode;
FIG. 2 is a schematic representation of an electrode coated with an
insulating dielectric material in a pattern defining raised areas
or ridges to be machined in a predrilled straight-walled hole, in
accordance with the principles of one embodiment of the present
invention;
FIG. 3 is a schematic representation of the hole shown in FIG. 2
after the raised areas have been formed with an electrode of the
present invention;
FIG. 4 is a schematic representation of an electrode of the present
invention that is solid, situated in a hole and includes a locator
to position the electrode in the hole;
FIG. 5 is a cross-sectional view through a locator;
FIG. 6 is a schematic representation of an electrode of the present
invention that is hollow, and situated in a blocked hole;
FIG. 7 is a schematic representation of another embodiment of an
electode in accordance with the instant invention; and
FIG. 8 is one embodiment of an electrode in accordance with the
instant invention.
DETAILED DESCRIPTION
A better understanding of the invention may be gained by reference
to the drawings. FIG. 1 is a schematic view illustrating a
conventional shaped-tube electrochemical machining (STEM) electrode
10 and the operation of electrode 10 in electrochemically machining
a hole 8 having interior walls 9 in an electrically conductive
workpiece 20. Electrode 10 of the prior art is typically a hollow
metal tube 11 coated on an exterior surface with an insulating
dielectric material 12 except at the end proximate to electrically
conductive workpiece 20, where a band 14 of exposed metal is
disposed. During the drilling operation, an electrolyte solution is
continuously circulated through the body of electrode 10 and hole 8
while an electrical potential is applied between electrode 10 and
workpiece 20. The solution is pumped to an inlet 16 at the end of
electrode 10 opposite the end composed of band 14 of exposed metal,
through the body of electrode 10, and through an end hole 18, which
end hole 18 is enclosed by band 14 of exposed metal, through hole 8
and out of the upper end of hole 8, to be collected in a sump (not
shown). The direction of electrolyte circulation is shown generally
by arrows 13 and 15.
Electric current passes between band 14 of exposed metal electrode
10 and a portion of wall 9 of hole 8 directly adjacent to band 14
of exposed metal, resulting in removal of metal from that area of
wall 9. Electrical insulation by dielectric material 12 blocks the
current flow in coated areas 17 on the exterior surface of
electrode 10, so that no deplating occurs in the areas of wall 9
opposite coated areas 17. The electrolyte solution dissolves the
deplated metal and carries the dissolved metal out of hole 8.
Because of the geometry of the exposed conductive surface of
electrode 10, a current is established primarily in a lateral
direction toward wall 9. Current density decreases as the distance
between wall 9 and band 14 of exposed metal of electrode 10
increases due to material dissolution, limiting the depth drilled.
In addition, operating conditions such as total machining time,
pulse amplitude, pulse on-time, and pulse off-time determine the
total electrical charges passing through the machined areas, which
operating conditions in turn determine the amount of metal removal.
As is known, these parameters, along with the nature and
concentration of the electrolyte and the operating voltage
determine the diameter of hole 8.
The conventional method of forming raised areas such as ribs or
ridges in hole 8 is to remove metal from areas of hole 8 adjacent
to the desired location of the raised area to form bulbs 32 by a
modified shaped-tube electrochemical machining (STEM) process. The
cyclic dwelling method of the prior art uses a cyclically varying
feed rate to form bulbs 32 of diameter greater than that of the
straight portion 30 of the hole. FIG. 1 shows the cyclic dwelling
method schematically. The feed rate is relatively fast when
drilling straight portion 30 of the hole, and relatively slow when
drilling bulbs 32. Similarly, cyclic variation of voltage can cause
formation of bulbs, or enhance the bulbing process. However, cyclic
variation of voltage requires a sophisticated power output.
The electrode and methods of the present invention provide for
convenient, cost effective machining of features especially in
holes with large aspect ratios. Examples of the features that may
be produced are turbulators in cooling holes in turbine airfoils,
rifling in gun barrels, and grooves in air bearing shafts.
With the improved electrode and machining process of the invention,
it is possible to machine as many bulbs as desired, in whatever
configuration desired, while achieving a significant reduction in
process time. Furthermore, no variation of process parameters such
as feed rate or voltage are needed; therefore, costly sophisticated
controls for the instrument are not required.
FIG. 2 depicts an electrode 100 in accordance with one embodiment
of the invention in a predrilled hole 101 having a straight wall
102, of an electrically conductive workpiece 110. FIG. 3 shows
electrode 100 in the same hole 101 after bulbs 120 and intervening
raised areas, or ridges 122, have been created. In the embodiment
shown in FIGS. 2 and 3, electrode 100 comprises a hollow
cylindrical electrically conductive cylinder 105 coated with an
electrically insulating coating 103 in a pattern having intervening
areas 104 of exposed metal or conductive material on the exterior
surface. The pattern of insulating coating 103 defines raised areas
or ridges to be machined in predrilled hole 101. In this
embodiment, the pattern is a series of rings 106. The (+) and (-)
designations indicate pulsed voltage through the body of electrode
100 and workpiece 110.
As shown in FIG. 3, areas of exposed conductive material 104 on the
surface of electrode 100 define areas where bulbs 120 are formed by
removal of metal from wall 102 of hole 101. Raised areas or ridges
122 are created in wall 102 of hole 101 where no deplating occurs
in the vicinity of insulated portions 106 of the surface of
electrode 100.
FIGS. 2 and 3 depict an embodiment of the invention where electrode
100 consists of cylinder 105, having a body composed of an
electrically conductive material. The diameter of cylinder 105 may
be as small or as large as necessary to fit the predrilled hole.
However, the outside diameter of cylinder 105, measured over the
coated surface, typically ranges between about 1 mm to about 8 mm.
The thickness of coating 103 is typically in the range between
about 0.15 to about 0.2 mm thick.
Cylinder 105 allows for pumping of an electrolyte solution into
hole 101 through an inlet 112 at the end of electrode 100 extending
outside hole 101 and out of end hole 114 at the other end of
electrode 100. Inlet 112 and end hole 114 facilitate uniform
electrolyte flow through the areas being machined. Electrode 100
may also have electrolyte outlets 116 along the exposed surface of
electrode 100. Outlets 116 in addition to end hole 114 may be
desirable where relatively large areas are being machined. The size
of outlets 116 determines the added amount of electrolyte supplied
to machining areas, which in turn determines surface quality of the
bulbs 120 as well as metal removal uniformity.
The operation of a shaped-tube electrochemical machining (STEM)
instrument with an electrode of the present invention is similar to
that with a conventional electrode. Current is provided by coupling
electrode 100 to a negative terminal of a STEM power supply (not
shown) and workpiece 110 to a positive terminal. Electrode 100 is
positioned inside smooth-walled hole 101 obtained from a previous
drilling step. An electrolyte solution, which solution may be the
same electrolyte as used in the first drilling step, is pumped into
an end of hole 101 under pressure. Where electrode 100 is hollow
and may contain outlets 116 for the electrolyte, the solution is
pumped into inlet 112 of electrode 100. In this embodiment, the
electrolyte flows into inlet 112 and out through outlets 116 along
the side surface of electrode 100 and end hole 114. All raised
areas or ridges as defined by the pattern of the coating of
electrode 100 may be formed in hole 101 simultaneously.
The body of electrode 100 of the invention is composed of a
conductive material, preferably titanium because of titanium's
resistance to electrolytic action. The outer surface of the
electrode body is covered with an electrically insulating coating
103 in a pattern that leaves some areas of the surface exposing the
conductive material of the body. Coating 103 is made of a
dielectric material, which dielectric material should preferably be
smooth, of even thickness, tightly adhered to the surface of the
body and free of pinholes or foreign material. Exemplary dielectric
materials suitable for electrode 100 of the present invention
include polyethylene, polytetrafluoro-ethylene, ceramics, and
rubbers.
The pattern in coating 103 on the electrode body of the present
invention defines raised areas or ridges 122 to be formed in
predrilled hole 101. A preferred pattern is at least one ring 106
or band circumferentially disposed on the external surface of
electrode 100. A more preferred pattern is a series of rings or
bands 106 circumferentially disposed on the external surface of
electrode 100. The present invention, however, contemplates
employing any pattern configuration desired. Examples of other
configurations that may be employed are lines, rings or bands
longitudinally disposed along the external surface of electrode
100. Additional configurations that may be employed are steps or
staircases, and one or more spirals or helices. The geometric
components of the pattern may also be disposed orthogonally or
obliquely, relative to a longitudinal axis 107 of electrode
100.
FIG. 4 illustrates another embodiment of the invention where an
electrode 140 is solid and may include a locator 144 at one end.
The function of locator 144 is to position electrode 140 in hole
101 properly, such that electrode 140 is coaxial with the walls of
hole 101. Locator 144 is preferably composed of the same
material(s) as an insulating coating 141 in other areas on the
exterior surface of electrode 140, differing only in the thickness
of coating 141. The outside diameter of electrode 140 measured at
locator 144 is less than the inside diameter of hole 101. This
outside diameter should be sufficiently small that electrode 140
may be easily inserted in hole 101, but sufficiently large so that
electrode 140 fits snugly within hole 101. Locator 144 preferably
comprises a coating of greater thickness compared to coating 141 on
other parts of electrode 140. For example, the thickness of the
coating 141 is typically in the range between about 50 to about 75
microns, while locator 144 typically comprises a thickness in the
range between about 100 to about 150 microns.
FIG. 5 depicts a cross-section of a locator 150 in a non-circular
hole 151. Locator 150 should have at least three points on a
surface in contact with wall 154 of hole 151, and should allow for
free flow of electrolyte through hole 151. Exemplary locator 150
has four arms 152 in contact with wall 154 of hole 151. Electrolyte
flows through spaces 156 between arms 152. No metal is exposed
between arms 152.
A locator is preferably disposed near the end of electrode 100
inserted in hole 101. Where the cross section of hole 101 is not
circular, it may be desirable to provide additional locator(s) 145,
to aid in centering electrode 100 in hole 101. A preferred position
for such an additional locator 145 is at a midsection of electrode
100 as shown in FIG. 6.
The electrode and method of the invention may be used with a
workpiece having blind (i.e. non-through) holes or through holes.
As described above, uniform electrolyte flow is important for
ensuring surface as well as metal removal uniformity. In one
embodiment of the invention, uniform electrolyte flow through a
blind hole is provided for. This is illustrated in FIG. 3. The
electrolyte solution is preferably passed through the interior of a
hollow electrode 100, into hole 101 and out of the opening at the
upper end of hole 101 and is collected in a suitable sump (not
shown).
For through holes, or holes with more than one opening, some
measure is preferably taken to ensure uniform electrolyte flow
inside hole 101. Through holes are commonly used in gas turbine
blades. For example, the cooling holes that are frequently
manufactured in such blades using shaped-tube electrochemical
machining (STEM) have an inlet and an outlet for the flow of
coolant.
One method to ensure uniform electrolyte flow in a through hole is
to block one end of the hole. FIG. 6 illustrates this method, with
a through hole blocked with a plug 162 of suitable material, for
example, rubber. Using this method, the electrolyte solution may be
passed through a hollow electrode 100 such as that depicted in FIG.
2 and 3. The outlet(s) for the solution may be located either along
the side or at the lower end of electrode 100. Where the electrode
is solid and the predrilled hole is a through hole, electrolyte
solution may be pumped in one end of the hole and out the other
end.
FIG. 4 shows the second method to ensure uniform electrolyte flow
in a through hole where the electrode is solid. Electrode 140
consists of a solid body 145 coated with a suitable dielectric
material 141 in a pattern, leaving areas where electrically
conductive material of the body is exposed, and a locator 144.
Using this method, electrolyte is pumped, for example, from the
lower end of hole 101, around electrode 140, and out of the upper
end of the hole 101.
EXAMPLE
A straight-walled hole was drilled in a workpiece made up of two
pieces of stainless steel clamped together. The hole was drilled at
the interface where the two pieces were joined using a standard
STEM apparatus and a conventional electrode similar to that shown
in FIG. 1. After the straight drilling was completed, an electrode
according to the present invention, such as that illustrated in
FIGS. 2 and 3, was connected to the STEM apparatus, and placed
within the predrilled hole. A set of bulbs was simultaneously
electrochemically machined in the hole, leaving raised areas, or
ridges, between the bulbs. The spacing of the rings of insulating
material in the pattern on the electrode correlated with the
spacing of the ridges in the hole, and the width of the rings
correlated with the width of the ridges.
The new process as discussed above uses a specially-designed,
partially-insulated electrode as a cathode to generate turbulators
with electrochemical dissolution an acid electrolyte and a pulsed
power supply. This process works well for making a variety of
turbulator geometrical features in different sizes of cooling
holes. For holes with very large aspect ratios, however, (for
example, 50 or above), that is, extra-long, thin holes, the small
hole diameter limits the diameter of the electrode. Accordingly,
the electric resistance of the electrode, typically made of a
titanium alloy, increases significantly along the length of the
electrode, which, in turn, causes significant machining voltage
drop along the length. Since metal removal rate is approximately
proportional to the machining voltage between the electrode and the
hole, an uneven metal removal along the electrode would occur and a
gradual tapering can be observed in the turbulated holes. There is
a need to minimize the tapering and improve the uniformity of the
turbulators over the hole length.
As shown in FIG. 8, another embodiment of an electrode 500 is made
of three types of materials. An outer metal skin 502 of electrode
500 is made of a corrosion-resistant material such as a titanium
alloy, for example, Ti A40, Ti 64, or Ti 6242. Corrosion-resistant
outer metal skin 500 increases the service life of electrode 500,
which electrode 500 is exposed to acid electrolyte during the
electrochemical machining. A core 504 of electrode 500 is made of
an electrically highly conductive material, such as copper,
aluminum, silver, or the like. An insulating coating 506 deposited
on outer metal skin 502 is an insulating material such as a
polymer, enamel, ceramic or the like.
There are many ways of fabricating the two metallic structures of
electrode 500. One of the methods is to use a tube made of a
corrosion-resistant material as mentioned above as outer metal skin
502 and compactly fill the tube's hole with a conductive core 504
material such as copper. An exemplary method of filling the hole is
to use an OFHC Cu wire that has a slightly smaller diameter than
the tube inner diameter and a couple of inches longer than the
tube. One end of the Cu wire is held in a chill block to prevent
melting of the length of the wire. With the end exposed for 1/8",
heat is applied with a GTAW (gas tungsten arc weld) torch to above
the melting temperature of the Cu. The end is melted back to the
chill block and heat is removed, leaving a round bead. The size of
the bead is controlled by the distance of the end of the wire above
the chill block. A bead with a diameter larger than the tube hole
diameter allows the annealed Cu to be lightly forced into the open
end of the Ti tube. The Cu wire is slid into the Ti tube until the
bead contacts one end of the tube and is lightly forced into the
end, closing it off. The Cu wire being longer than the tube, sticks
out past the other end. The uncoated shank of the tube allows the
wire and the tube to be close enough together to be resistance
welded to keep the two parts together with adequate electric
contact. This method is applicable to both the tubes with an
insulated coating and without coating.
Another method is to fill the tube hole with a highly conductive
alloy with a low melting point. After filling, the tube can be
heated to allow the alloy to compactly stuff the hole to provide
the needed high conductance.
This fabrication method can also be done before the tube is coated
with insulation layer. In this case, copper pack powder can be
filled in the hole and then be heated to copper meting point to
ensure compactly filling.
This invention has been tested for making a turbulated holes with
100 mils of diameter and 7.5 inches of length. Uniform turbulator
geometry has been obtained.
While only certain features of the invention have been illustrated
and described, many modifications and changes will occur to those
skilled in the art. It is, therefore, to be understood that the
appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the invention.
* * * * *